Question re handling of MAVLINK RC_CHANNELS_OVERRIDE messages

I am sending an RC_CHANNELS_OVERRIDE MAVLINK message where I want to override channels 0-13 from MissionPlanner to AP. I see that NUM_RC_CHANNELS is defined to be 16 in RC_Channel.h, but the code in GCS_Mavlink.cpp:handleMessage() only uses channels 0-7. Is there a reason for this? What other MAVLINK message can I use to force AP to let me override more that the first 8 RC Channels? Thanks.